A turbomachine is for example a conventional turbocompressor 101, as is shown in FIGS. 8 and 9. The turbocompressor 101 has a casing 102 and a turbocompressor rotor 103 which is enclosed by the casing 102. The turbocompressor rotor 103 has a shaft 104 which is supported on the casing 102 on a bearing point 105. In addition, the turbocompressor rotor 103 has an impeller 106 which is arranged in an overhung manner in relation to the bearing point 105. The impeller 106 is a centrifugal compressor impeller, the inflow of which runs in the axial direction of the shaft 104 and the outflow of which runs in the radial direction of the shaft 104. The inflow is guided in an inlet passage 107 of the turbocompressor 101 which extends in the axial direction of the shaft 104 so that the inflow impinges upon the impeller 106 in the axial direction of the shaft 104. For manipulating the inflow, for example for imposing swirl upon the inflow, a multiplicity of variable guide vanes 108 are arranged in the inlet passage 107, wherein the variable guide vanes 108 are assembled to form a blade ring.
As is shown in FIG. 8, the hub region of the guide vanes 108 is provided with a nose dome 110 which is fastened on the impeller 106 and therefore rotates along with the impeller 106.
The nose dome 110 is aerodynamically formed and serves for minimizing as far as possible the disturbing influence of the hub region of the impeller 106 in the inflow.
As is shown in FIG. 9, alternatively to the rotating nose dome 110 a stationary nose dome 111 can be provided in the region of hub region of the impeller 106. The nose dome 111 is held in position by a strut 109 which is fastened in the inlet passage 107. The nose dome 111 is also aerodynamically formed so as not to disturb as far as possible the inflow to the impeller 106. Furthermore, the strut 109 is aerodynamically formed in order to minimize as far as possible the disturbing influence of the strut 109 upon the inflow. The strut 109 is arranged upstream of the variable guide vanes 108 and the stationary nose dome 111 is formed longer in the axial direction of the shaft 104 than the rotating nose dome 110. Between the impeller 106 and the stationary nose dome 111 a relative movement takes place during operation of the turbocompressor 101. The nose dome 111 is arranged at a distance from the impeller 106 so that the impeller 106 does not come into contact with the stationary nose dome 111 and therefore cannot rub upon it and damage it.
The rotor dynamics of the turbocompressor rotor 103, i.e. the dynamic vibrational behavior of the turbocompressor rotor 103 during operation of the turbocompressor 101, depends essentially upon the geometry and the constructional design of the turbocompressor rotor 103 and upon the dynamic characteristics which are inherent to the bearing point 105, especially the rigidity and the damping of the bearing point 105. The bearing point 105 is conventionally fanned by a tilting-pad slide bearing which has the corresponding rigidity and the corresponding damping in conformance with its design, construction and its operation. When running up the turbocompressor rotor 103 to its operating speed the turbocompressor rotor 103 as a rule passes at least one critical speed. In order to avoid damage of the turbocompressor rotor 103 when running up to the critical speed, the radial vibration amplitude of the turbocompressor rotor 103 has to lie constantly within constructionally specified limits. These limits can be maintained when the rotor dynamics of the turbocompressor rotor 103 are correspondingly established, especially by means of the geometry and the constructional design of the turbocompressor rotor 103 and of the rigidity and the damping of the bearing point 105.
High radial vibration amplitudes which are induced by the overhung support of the impeller 106 are frequently to be recorded on the impeller 106 when running up to the critical speed. As a result, the turbocompressor rotor 103 with its overhung-supported impeller 106 is frequently difficult to control from the rotor-dynamic point of view.
Moreover, it is known that especially in the case of high gas densities an interaction of the compressor rotor with the compressed gas can result in self-excited vibrations of the rotor which can lead to destruction of the machine. This phenomenon is countered by means of a rotor-dynamically favorable construction, wherein up to now only the speed and the mass distribution of the rotor and also the damping and the rigidity of the bearing are parameters to vary.
Turbomachines with nose domes are known from documents U.S. Pat. No. 4,772,135, U.S. Pat. No. 6,158,977 and U.S. Pat. No. 6,327,857.